A popularly employed method for controlling an electric power system is centralized control of an electric power system (for example, see Patent Document 1). Centralized control of an electric power system requires building of a mechanism for centralizing information and a large-scale control system for grasping and analyzing the whole system configuration condition and also optimizing the whole system. Such a system is easily applied in countries and regions where electric power supply infrastructure has developed to some extent, because the system can be built additionally. However, new building of the above system in regions including emerging countries where electric power infrastructure has not developed requires a large investment in development of the infrastructure.
On the other hand, in terms of optimization of the whole system, there is a proposed approach of distributed control, which is less efficient but allows electric power control without a large investment by regulating voltages between neighboring power plants or comparatively near power plants (for example, see Patent Document 2). In control of an electric power system, generally, matrix calculation is carried out on the basis of information of each generator, and a solution is found. In the case of concentrated control mentioned above, large-scale matrix calculation should be done because all the information is included. On the other hand, in the case of distributed control, sparse matrix calculation is done because control between neighboring or near power plants is executed, and therefore, the amount of calculation is considerably less.
In recent years, in consideration of the protection of global environment and energy security, natural energy such as solar light, biomass and wind power has been introduced worldwide. However, such natural energy has a problem with the stability of supply and, as such power sources become more popular in the future, it will become a critical issue how to keep the stability of the whole electric power network. In other words, when it comes to electric power generation by natural energy, new entrants to electric power supply increase because the introduction cost is low though the power generation capacity of each generator is small, and moreover, it is worried about that enough reserves cannot be secured in a small-scale electric power system configured mostly by such generators. When electric power supply from a system stops (blackout), a diesel generator may be used as a backup. However, in most cases, the diesel generator does not interconnect with the system, and this is not an idea of efficiently using distributed energy resources at all times.
As an example of the configuration of distributed control, a configuration in which a plurality of distributed energy resources are installed is disclosed in Patent Document 3. In this example, because a system with high balancing capability is connected at all times, the respective distributed energy resources can stably operate. A power distribution configuration connected to an electric power system and including distributed energy resources is shown in FIG. 1. As shown in FIG. 1, a distribution substation 1 connects to an electric power system 5, a plurality of generators 2 and a plurality of loads 4, and electric power supplied from the electric power system 5 and the generators 2 is supplied to the loads 4 and consumed thereby. When the balance between supply and demand of electric power is kept, the following equation 1 holds:
                              G_main          +                                    ∑                              i                =                1                            m                        ⁢            G_i                          =                              ∑                          j              =              1                        n                    ⁢          L_j                                    [                  Equation          ⁢                                          ⁢          1                ]            
where G_main denotes electric power supplied from the electric power system 5, G_i denotes electric power generated by an ith generator 2, L_j denotes electric power consumed by a jth load 4, m denotes the number of the generators 2, and n denotes the number of the loads 4.    Patent Document 1: Japanese Unexamined Patent Application Publication No. JP-A 2002-165367    Patent Document 2: Japanese Unexamined Patent Application Publication No. JP-A 2010-057311    Patent Document 3: Japanese Unexamined Patent Application Publication No. JP-A 2000-333373
Emerging countries and developing countries often face a situation that they cannot be supplied with electric power from an electric power system and, in this case, it is difficult to make a plurality of distributed energy resources stably operate while keeping the balance between supply and demand. In other words, because G_main on the left side of the equation 1 instantaneously becomes zero, generators need to balance supply and demand again, but there is a case where it is impossible to respond to sudden change or it is impossible to supply electric power consumed by all the loads in the first place. Moreover, when electric power supply from the electric power system stops or in a case where an electric power network is isolated at all times as in a remote island, high capability of balancing by the system cannot be expected.
In general, regulation of the amount of electric power generated by a generator in a short time period (from few seconds to few minutes) is autonomously executed by the generator in governor-free operation (referred to as governor control hereinafter). However, because of the failure to follow load change, it takes long time to make generators stable because there are a plurality of generators, or an unallowable abnormality in frequency and voltage occurs. In some cases, it escalates to a critical problem that a generator is disconnected, for example. In particular, when electric power supply from the electric power system stops, a great difference in supply and demand of electric power is instantaneously made, and it becomes a problem for control.
Further, in a case where real-time control is possible, a control device can send a command to each of the generators and forcibly control generated electric power in order to fill the difference between supply and demand of electric power, or can control the loads in order to limit demand. However, there is a need to install a fast and reliable communication network and a number of sensors and controlled devices, and there is a problem that installation cost is high particularly in emerging countries and developing countries. Therefore, a mechanism and control to allow a delay and an error in communication and autonomously perform stable balancing of supply and demand.